Permanent magnet system for the generation of at least two opposite magnetic fields lying one behind the other for the bundled guidance of an electron beam, especially for traveling wave tubes



PERMANENT MAGNET SYSTEM FOR THE GENERATION OF AT LEAST TWO OPPOSITE MAGNETIC FIELDS LYING ONE BEHIND THE OTHER FOR THE BUNDLED GUIDANCE OF AN ELECTRON BEAM, ESPECIALLY FOR TRAVELING WAVE TUBES Filed Jan. 21, 1966 2 Sheets-Sheet 1 March 12, 1968 P MEYERER 3,373,388

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March 12, 1968- P. MEYERER 3,373,388

PERMANENT MAGNET SYSTEM FOR THE GENERATION OE AT LEAST TWO OPPOSITE MAGNETIC FIELDS LYING ONE BEHIND THE OTHER FOR THE BUNDLED GUIDANCE OF AN ELECTRON BEAM, ESPECIALLY FOR TRAVELING WAVE TUBES Filed Jan. 21, 1966 2 Sheets-Sheet 2 N 2 w I 2 i w m m m m 1/ 1 1 INVENTOR Qm/ Meye er biz ATTYS.

United States Patent Office 3,373,338 Patented Mar. 12, 1968 9 Claims. oi. 335-210) ABSTRACT OF THE DISCLOSURE A permanent magnet array for a so-called reversed field focusing for travelling wave tubes with permanent magnets symmetrically arranged relative to the system axis which according to the invention are arranged in a closed series one behind the other, wherein the expansion of the magnets decreases uniformly and perpendicularly to the system axis from the middle of the closed series to its two ends, the magnets particularly consisting of a material with a high coercive force.

The invention relates to a permanent magnet system for the generation of at least two magnetic fields directed oppositely to one another and lying in succession in the longitudinal direction of the system, for the bundled guidance of an electron beam over a relatively long path interval, especially for traveling wave tubes, with permanent magnets symmetrically arranged relative to the system axis, which are magnetized in planes perpendicular to the system axis, and in which the poles turned away from the system axis are magnetically connected with one another by soft iron bridge members, which are provided, at least on the two end faces of the system, with soft iron parts extending toward the system axis, which form in each case a soft magnetic diaphragm or wall.

In electron beam tubes for the amplification and generation of very high frequencies, especially in traveling wave tubes, the electron beam, as is well known, must be conducted in bundled form over a relatively long path interval. For this purpose magnetic focusing devices are known, by means of which there is generated in the interior of the discharge vessel of the tube along the discharge path either a homogeneous or an alternating magnetic field.

There also are already known permanent magnet devices for the focusing of the electron beam of highestfrequency tubes, in which the electron beam is conducted in bundled form with the aid of several magnetic fields lying in succession in beam direction, which in each case are essentially homogeneous. This type of focusing, which is designated as so-called reversed field focusing, has the advantage that in principle the weight of the permanent magnet system concerned, with equal efiiciency with respect to the focusing properties, can be kept lower than in a focusing of the electron beam with a homogeneous magnetic field. A known arrangement for the generation of a corresponding reversal field consists of tubular permanent magnets which are arranged relative to one another with oppositely directed polarization. This permanent magnet arrangement, however, has at its exterior a strong magnetic scatter field which does not contribute to the bundling of the electron beam and, consequently, again requires an increase of the magnet weight. Moreover, the considerable external scatter field has to be magnetically shielded with respect to other adjacent highfrequency devices.

Another known arrangement for the bundled conduction of an electron beam of a traveling wave tube, with the aid of several homogeneous magnetic fields lying in succession in beam direction, consists of permanent magnets which are arranged between the ends of the delay line of the tube in at least one plane perpendicular to the electron beam. The poles of the permanent magnets facing away from the electron beam are connected by a soft iron sheet metal body extending along the delay line of the tube and somewhat beyond the delay line.This soft iron sheet metal body, which is disposed symmetrically with respect to the system axis, is closed off at its end face with soft plates which extend as far as the vacuum shell of the tube. In order to achieve a certain homogeneity of the magnetic field along the system axis the magnet poles adjacent to the electron beam and the soft iron plates mounted on the ends of the soft iron sheets metal body are magnetically connected with one another by a soft iron cylinder arranged coaxially with the beam axis. In consequence of the presence of the tubular soft iron cylinder, here too, a considerable part of the magnetic induction does not contribute to the focusing of the electron beam, whereby the individual permanent magnets must have an appreciable magnetic energy content, which corresponds to a high weight and great space requirement for the permanent magnets.

The invention has as its basic problem that of creating a permanent magnet system for the generation of at least two oppositely directed magnetic fields lying in succes sion in longitudinal direction of the electron beam (reversal field) for the bundled guidance of the electron beam of highest-frequency tubes, in which system the magnetic scatter flux is low in proportion to the useful flux. For the solution of this problem, in a permanent magnet system of the type initially mentioned, it is proposed according to the invention that the permanent magnets be disposed between two soft magnetic diaphragms or walls, and arranged one behind the other in a closed series with the magnetic potential of the permanent magnets decreasing from the middle of the closed series to its two ends, the poles of the permanent magnets adjacent to the system axis all being of like polarity.

In a permanent magnet system for the generation of a homogeneous magnetic field for the bundled guidance of an electrom' beam over a relatively long path interval it is a known practice, per se, to arrange, in succession, permanent magnets disposed perpendicularly to the system axis and magnetized in a closed series, in order to achieve a low weight of the magnet system. There, however, in contrast to the present invention, the magnetic potential of the permanent magnets uniformly diminishes from the two ends of the system toward the middle of the system. A further important difference of this known arrangement from a permanent magnet system according to the present invention lies in the feature that the poles of the permanent magnets adjacent to the system axis are of the same designation merely from the end faces of the system to the middle thereof.

A permanent magnet system according to the invention presents not only the advantage that the ratio of useful magnetic flux to magnetic scatter flux is very good, but moreover, through the spatial arrangement of the permanent magnets, the coupling and decoupling lines which in a traveling wave tube are ordinarily arranged at the two ends of the delay line, can be spatially well housed in the system without any particular difficulties. The permanent magnets themselves advantageously consist of a hard magnetic ferrite material which, as compared to the nickel-iron-cobalt alloys usual for permanent magnets, have a low specific gravity.

The permanent magnets of a system according to the invention are preferably so dimensioned that the magnetic potential of the permanent magnets decreases from the middle of the closed series to their two ends approximately proportionately with the path distance in longitudinal direction of the system. It is thereby achieved that the individual magnetic fields lying one behind the other are essentially homogeneous as is normally the case in a focusing with a reversal field. It is also possible, however, through appropriate choice of the potentials, to achieve a course of the magnetic induction in the individual magnetic zones, in particular increases in field strength if, in certain cases of application this is desired, whereby such course is not fully homogeneous.

The invention will be explained in detail with the aid of the drawings, wherein like reference characters indicate like or corresponding parts, in which:

FIG. 1 is a longitudinal section of a permanent magnet system, according to the invention;

FIG. 2 is a transverse section taken approximately on the line 11-11 of FIG. 1;

FIG. 3 illustrates the field course of the permanent magnet system of FIG. 1, and is oriented with respect to the latter whereby the values illustrated correspond to the respective points along the system axis;

FIG. 4 is a longitudinal section, similar to FIG. 1, of a modification of the invention;

FIG. 5 is a transverse section taken approximately on the line VV of FIG. 4; and

FIG. 6 illustrates the field course of the construction of FIGS. 4 and 5, and is oriented with respect to FIG. 4.

Referring to FIGS. 1 and 2, the reference numeral 1 designates block-shaped permanent magnets which are magnetized perpendicularly to the longitudinal axis of the magnet system and consist preferably of a material of high coercive strength (H l50O oe.), for example, a hard magnetic ferrite. The permanent magnets 1 are arranged one behind the other in four closed series, which symmetrically surround the system axis. The ends of permanent magnets 1 turned away from the system axis lie in a common plane, while the ends of the permanent magnets adjacent to the system axis are offset stepwise with respect to one another. The graduation is so carried out that the magnetic potential of the permanent magnets 1 uniformly decreases from the middle of the closed series to their two ends, in which arrangement all of the poles of the permanent magnets adjacent to the system axis are of like polarity. For the clarification of such magnetic potential relations there appear in the individual permanent magnets 1 numbers with a positive sign which designate a normalized value of the particular magnetic potential. The poles of the permanent magnets 1 turned away from the system axis engage soft iron bridge members 2 which are connected with one another to form a shield casing having a square cross section. Such shielding casing is provided at its end face with soft iron plates which extend toward the system axis and there form in each case a soft magnetic diaphragm or wall 3. By means of two diaphragms 3 there is produced along the axis of the system a magnetic field with a reversal of the magnetic induction in the middle of the system. Beneath FIG. 1 there is represented in FIG. 3 the field course which is achieved in a permanent magnet system according to FIGS. 1 and 2. The abscissa of the rectangular coordinate system represents the path z in longitudinal direction of the system and the ordinate the magnetic induction B The solidly drawn curve 4 represents the value of the magnetic field B along the axis of the focusing system according to FIGS. 1 and 2 with the magnetic potentials there indicated, such magnetic field being a reversal field with two magnetic fields oppositely directed extending one behind the other, which are in each case essentially homogeneous. The broken-line curve 5 indicates that in the individual zones of the reversal field the course of the magnetic field strength on the system axis need not be homogeneous. This can be brought about in a permanent magnet system according to FIGS. 1 and 2 by an arrangement in which the steps between adjacent permanent magnets 1 are not uniformly reduced along the path interval z in a direction perpendicularly to the system axis.

FIGS. 4 and 5 illustrate another example of construction of a permanent magnet system according to the invention, in which the permanent magnets 1, as seen in longitudinal direction of the system, form two closed series, between which there is arranged a soft-magnetic diaphragm or wall 3, which extends from the soft iron bridge members 2 perpendicularly toward the system axis. The permanent magnets 1, again in block form, are in each case arranged between two adjacent magnetic diaphragms 3, symmetrical to the system axis, in two closed series 0pposite one another. The permanent magnets 1, as in the example of construction of FIGS. 1 and 2, are so offset stepwise with respect to one another from the middle of the respective closed series to the two ends thereof that the magnetic potential of the permanent magnets uniformly diminishes in longitudinal direction of the system toward each end of the associated series. The ends turned away from the system axis of the permanent magnets 1 likewise lie in a common plane and are magnetically connected with one another by the soft iron bridge members 2. The soft iron bridge members 2 are provided with the soft iron plates which form the soft magnetic diaphragms 3 and bring about a reversal of the magnetic induction.

The course of the magnetic induction achieved with a system according to FIGS. 4 and 5 is represented beneath FIG. 4 in FIG. 6, in which the curve 6 represents the value of the magnetic field B along the axis 2 of the magnet system. It is recommended that a corresponding magnet system be used if an electron beam is to be guided in bundled form over a very great length, for example up to 20 cm. The necessary magnetic weight is then less than if the guiding is effected with only one reversal field in which, as illustrated in FIG. 3, only two magnetic fields are arranged one behind the other.

The permanent magnet systems illustrated in FIGS. 1 and 2 and in FIGS. 4 and 5 are magnetically completely shielded, because the magnetic circuit is closed by the soft iron yokes or bridge members disposed on the side faces. In consequence of the very low magnetic dispersion flux, the permanent magnets 1 may consist of a low energy magnetic material with high coercitive strength, for which purpose there is especially suited a hard magnetic ferrite material. As an example, there should be mentioned a ferrite which has a field strength of 1800 0e. and a magnetic induction of 1800 G. With such a ferrite a maximal magnetic induction is attainable along the system axis of 900 G.

The invention is not limited to the examples of construction illustrated. For example, in certain cases the permanent magnets which are adjacently arranged in closed series can also be of ring-shaped configuration.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. A permanent magnet system for the generation of at least two magnetic fields, oppositely directed to one another, and lying one behind the other in longitudinal direction of the system, for the bundled guidance of an electron beam over a relatively long path interval, especially for traveling wave tubes, comprising a plurality of permanent magnets arranged symmetrically to the longitudinal axis of the system, said magnets being magnetized perpendicularly with respect to the system axis, soft iron bridge members disposed at the respective poles of the permanent magnets facing away from the system axis magnetically connecting such poles, and soft iron parts disposed at least at the two ends of the system, which in each case form a soft magnetic diaphragm, said permanent magnets being arranged in a closed series, one behind the other between two of said adjacent soft magnetic diaphragms, said permanent magnets being so arranged with respect to the system axis that the magnetic potential of such magnets diminishes from the middle of the closed series toward the two ends thereof, all of the poles of said magnets facing the system axis being of like polarity.

2. A permanent magnet system according to claim 1,

wherein the permanent magnets decrease in their extent erpendicularly to the system axis from the middle of the closed series toward its two ends.

3. A permanent magnet system according to claim 2, wherein the ends of the permanent magnets facing away from the system axis lie in a common plane, while the ends of the permanent magnets adjacent to the system axis are offset stepwise relative to one another.

4. A permanent magnet system according to claim 3, wherein the permanent magnets, which in each case form a step, are of block formation.

5. A permanent magnet system according to claim 4, wherein the permanent magnets are arranged, in each case, between two adjacent soft magnetic diaphragms in four closed series which symmetrically surround the system axis.

'6. A permanent magnet system according to claim 4, wherein the permanent magnets are arranged, in each case, between two adjacent soft magnetic diaphragms in two closed series opposed to one another and symmetrically disposed relative to the system axis.

7. A permanent magnet system according to claim 1, wherein the permanent magnets are so disposed that the magnetic potential of the respective magnets uniformly decreases from the middle of the closed series toward their two ends in such a way that the magnetic fields lying one behind the other in longitudinal direction of the system are, in each case, homogeneous.

8. A permanent magnet system according to claim 7, wherein the permanent magnets, as viewed in longitudinal direction of the system, form two closed series, 'between which there is arranged a soft magnetic diaphragm which extends from the soft iron bridge members perpendicularly toward the system axis.

9. A permanent magnet system according to claim 1, wherein the permanent magnets consist of a material of high coercitive strength, in particular a hard magnetic ferrite material.

References Cited UNITED STATES PATENTS 3,205,415 9/1965 Seki et a1. 335-210 FOREIGN PATENTS 77 0,133 3/ 1957 Great Britain.

BERNARD A. GILHEANY, Primary Examiner. GEORGE HARRIS, JR., Assistant Examiner. 

